A horizontal axis wind turbine is illustrated in FIG. 1 to which reference should now be made. Schematic FIG. 1 illustrates a wind turbine 1, comprising a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising at least one wind turbine blade 5 is mounted on a rotor hub 6.
The hub 6 is connected to the nacelle 3 through a shaft (not shown) extending from the nacelle front. The nacelle 3 can be turned, using a yaw drive positioned at the top of the tower 2, to change the direction in which the rotor blade hub 6 and the blades 5 are facing. The blades are aerodynamically profiled so that they experience a ‘lift’ or pressure from the wind as the wind flows past the surface of the blade. The angle or pitch at which the leading surface of the blade aerodynamic profile meets the incident wind can be altered using a pitch drive, which turns the blades 5 with respect to the hub 6 thereby adjusting the “lift” achieved by the blades and thereby also the rotational driving force on the rotor for any given wind speed.
The wind turbine illustrated 1 in FIG. 1 may be a small model intended for domestic or light utility usage, or it may be a larger model. Some large models in particular may be installed in large scale electricity generation on a land based or offshore wind farm. A typical commercial wind turbine, for example one that is designed to generate say 3 MW of power, can stand approximately 100 meters high and have wind turbine blades with a length of around 40 m or more. The size of the wind turbine blade, and in particular the area that is swept out by the blades as they turn in the wind is linked to the amount of energy the turbine can extract from the wind. In commercial energy generation wind turbines are therefore large so that they provide the greatest generation capacity. On- and offshore wind turbines are known with rotor diameters in the range between 140 and 180 metres. Some of these models can generate around 8 MW of power. Tower heights above 80 or 100 metres are becoming increasingly commonplace, even extending to heights of 140 metres or more for very large turbines or for turbines whose rotors need significant ground clearance in order to avoid possible local turbulence effects in wind passing close to the ground.
In normal operation such as during power generation, the yaw drive turns the nacelle 3 so that it points the rotor hub 6 into the wind and the pitch drive adjusts the blades 5 of the wind turbine so that they are positioned with an angle of attack which creates lift and causes the rotor 4 to turn. The pitch of the blades can then be adjusted so that they force they experience from the wind is maintained within safe operating parameters, while generating as much energy from the incident wind as possible.
A turbine tower 2 serves multiple purposes. First, it provides a yaw and support platform for a nacelle 3 and blade rotor 4 at such a height that the rotor is clear of the ground and is as much as possible positioned in laminar air currents at the installation location. As such, the tower must possess the requisite structural strength to sustain the rotor and nacelle in position, even under extreme wind or weather conditions. Moreover, considering the dynamic nature of the turbine and of the weather, the tower must be able to perform its functions in spite of mechanical vibrations or oscillations visited on it and this, for the lifespan of the turbine 1 which may be twenty, twenty five or more years. A tower 2 may provide, near its base, an anchoring region positioned and fixed on a foundation. Foundations on land may typically consist in a mass of cast concrete embedded in the land surface or placed on rock formations, into which reinforcing structures, connectable to the tower base may extend. Offshore foundations can take a wide variety of forms. Two common forms include monopile and jacket foundations which latter type resemble a platform construction. In certain constructions, especially in offshore tower constructions, the tower 2 may include a base region connectable or connected to a foundation such as a monopile foundation or jacket foundation and sometimes known as a transition piece. In addition, the tower 2 houses certain wind turbine components, typically those elements which relate to turbine control, servicing or to power transmission from the turbine to the grid. These parts along with the nacelle or rotor etc. need to be serviced from time to time, thereby necessitating access by service personnel. The need to provide access into a tower 2 for personnel can pose a problem in relation to the structural properties of the tower wall. This is especially so since the access locations for service personnel are likely to be at the tower base, namely precisely at the location where the highest loads and stresses act on the tower.
In WO2009094991 there is disclosed a substantially oval doorframe in a tower wall, within which doorframe is arranged a casement incorporating a doorway with a door and also incorporating two ventilation apertures with venting elements, one above and one below the doorway. All these elements are arranged within the extent of the doorframe which is arranged in the form of a coaming through the tower wall, extending both internally and externally of the wall. The doorframe coaming is substantial and oval shaped (when seen in plan view) in order to mitigate for weakening of the tower wall by the provision of access therethrough.
In large turbines, especially offshore turbines and larger land-based turbines, some components located in the tower can be very large and bulky, and sometimes extremely heavy: of the order of several tonnes or tens of tonnes. Moreover, in very large turbines such as offshore turbines, some components which would ordinarily be placed in the nacelle 3 may be too large or too heavy for the nacelle, and may therefore instead be positioned inside the tower 2, sometimes at its base region, possibly in a transition piece region of a tower. Thus, in addition to service personnel requiring access to the tower 2 or turbine 1 from time to time, larger towers 2 may advantageously also be dimensioned to allow large turbine components to be brought inside the tower, and should preferably also allow occasional removal of large components for servicing, repair or replacement. In view of the need to move large elements in or out through the tower, it can be useful to provide a correspondingly large access panel.
In DE102008035350, it has been suggested to provide an access aperture through a tower wall, which aperture is sufficiently large to allow introduction or removal of a wind turbine transformer. To that end, there is proposed a closure plate, larger than the aperture, and removably bolted to the tower wall by means of holes around the plate edge and all around the tower aperture edge. The closure plate encompasses a heavily thickened doorframe portion for an additional, smaller, personnel entrance door.
The size of an access panel can vary although in larger turbines, these may extend in a height dimension up to around six or eight metres or more with a width of up to two metres or more. As such, these panels, being large and therefore also heavy can pose a challenge to put in place or to remove when required. Weather considerations may also need to be taken into account such as winds, which can preclude or at least seriously complicate application (closure) or removal (opening) operations. As such, the need both to support and to accurately locate an access panel such as shown in DE102008035350 or in WO2009094991, at its corresponding tower aperture is difficult and in some conditions, impossible. In offshore locations, the operation may be more difficult still.
There remains a need to provide access to the inside of a wind turbine tower, even while avoiding weakening of the tower structure. Moreover, there is a need for improved convenience relating to access through the tower wall for moving large components or equipment.